Biomass refining (or biorefining), which separates cellulose, hemicellulose, and lignin from biomass feedstocks, is becoming more prevalent in industrial plants. Cellulose fibers and sugars, and hemicellulose sugars, are being used by many companies for chemical and fuel production. Indeed, we now are observing the commercialization of integrated biorefineries that are capable of processing incoming biomass much the same as petroleum refineries now process crude oil. Underutilized lignocellulosic biomass feedstocks have the potential to be much cheaper than petroleum, on a carbon basis, as well as much better from an environmental life-cycle standpoint.
One of the biggest and well-known challenges in many biorefineries is dealing with lignin. Lignin is a major component of biomass. It is typically between 15-35 wt % (dry basis) of the biomass material. Lignin has good fuel value, similar to some types of coal.
The word lignin is derived from the Latin word “lignum” meaning wood. Lignin is a natural polymer and is an essential part of wood and other forms of cellulosic biomass, including agricultural crop residues such as sugarcane bagasse. Lignin performs multiple functions that are essential to the life of the plant, including transport of nutrition and durability of the biomass. Lignin imparts rigidity to the cell walls and acts as a binder, creating a flexible composite cellulose-hemicellulose-lignin material that is outstandingly resistant to impact, compression, and bending.
After polysaccharides (polymers of sugar), lignin is the most abundant organic polymer in the plant world. Lignin is a very complex natural polymer with many random couplings, and therefore lignin has no exact chemical structure. The molecular structure of lignin consists primarily of carbon ring structures (benzene rings with methoxyl, hydroxyl, and propyl groups.
Various processes can be used to remove and isolate lignin from biomass. Each process, however, produces material of different composition and properties. Generally there are four important factors to take into account when working with lignin:
1. Source of the lignin.
2. Method used to remove lignin from the biomass.
3. Method(s) used to purify the lignin.
4. Nature of the chemical modification of the lignin after isolation.
These factors influence the properties of the lignin. Important properties of lignin formulations include molecular weight, chemical composition, and the type and distribution of chemical functional groups.
Separation and recovery of lignin is quite difficult. It is possible to break the lignin-cellulose-hemicellulose matrix and recover the lignin through a variety of treatments on the lignocellulosic material. However, known lignin recovery methods generally have one or more important commercial-scale limitations. Lignin purification from biomass is a classic chemical-engineering problem with complex chemistries and transport phenomena, criticality of reactor design and scale-up, serious analytical challenges, and many practical issues arising from lignin's propensity to stick to equipment and piping.
Lignin can be difficult to process in biorefineries because it has a tendency to deposit on solid surfaces and cause plugging. Although lignin handling has always been known to be a challenge, there remains a need in the art for ways to either avoid lignin precipitation or to deal with it after it occurs. Other difficulties are caused by downstream fermentation inhibition caused by lignin, as well as lignin fragments and derivatives (e.g., phenolics, acids, and other compounds).
Lignin separations challenges appear to be particularly troubling problem for acidic pretreatments of biomass or biomass-derived liquors. For example, in van Heiningen et al., “Which fractionation process can overcome the techno-economic hurdles of a lignocellulosic biorefinery,” Proceedings of the AIChE Annual Meeting, Minneapolis, Minn. (2011), it is cautioned that “an operating problem which has mostly been overlooked for acidic pretreatment is formation and precipitation of sticky lignin on reactor walls and piping.” The lack of R&D attention to this problem is stated to be that it only “becomes apparent in continuous larger scale operation after one to two week operation.”
In view of the aforementioned needs in the art, improvements are clearly needed to deal with lignin precipitation during acidic hydrolysis of biomass and/or biomass hydrolysates (such as hemicellulose-containing liquid extracts).